Research progress on microbes in aerobic granular sludge

Abstract

Aerobic granular sludge (AGS) is a mobile biofilm which is formed by varied microbial populations by self-coherence under certain selective pressure. Aerobic granular sludge technology has merits such as high sludge concentration, little residual sludge, fast settling velocity, strong shock resistance, small area requirement, low cost, low energy consumption, simultaneous nitrogen and phosphorus removal, which has become a research hotspot in wastewater treatment. There have been a lot of research and discussion on the characteristics, influencing factors, application and formation mechanisms of AGS. It is beneficial to the formation of AGS by reducing the settling time, increasing the hydraulic shear force and the concentration of dissolved oxygen, and using low load alternating operation, adding calcium and magnesium salt. As a result, it is not conducive to the growth of some kinds of microorganisms, but favors the aggregation of the filamentous bacteria and other bacteria to form a symbiotic system, namely, granular sludge. This article discusses the three aspects of composition, distribution and succession of microbial communities from the microbial perspective of aerobic granular sludge, and the future research directions are prospected, which provides references for further research of aerobic granular sludge. Generally speaking, qPCR, FISH and T-RFLP are used for the analysis of a few kinds of microorganisms, and the PCR based cloning library, DGGE and high-throughput sequencing methods are used to analyze the complex microbial communities in the system. Metagenomics and metatranscriptomics analyses are the latest methods on microbial ecology. Extreme environment, heavy metal and nano materials have different effects on the microbial composition of AGS. As the framework of AGS, filamentous bacteria plays an important role on the AGS self-aggregation and morphology. A low value of pH or DO, insufficient or high organic loading will lead to excessive growth of filamentous bacteria in AGS, resulting in AGS deposition performance degradation, microbial AGS on filamentous bacteria composition research. The main conclusions are as follows: (1) microbial compositions of aerobic granular sludge differs from activated sludge, microorganisms as γ - Proteobacteria, Flavobacteria , Sphingobacteriia are remarkable in aerobic granular sludge; (2) study on efficient bacteria in nitrogen and phosphorus removal mainly including Accumulibacter , Nitrosomonas and Nitrospira ; (3) denitrifying phosphorus removal for aerobic granular sludge is the results of interaction of Competibacter and Accumulibacter , as well as Xanthomonadales and Tetrasphaera ; (4) low pH, low DO and high organic load will lead to excessive growth of filamentous bacteria such as Chloroflexi, Sphaerotilus and Thiothrix in aerobic granular sludge; (5) Rhodocyclus , Hyphomicrobiaceae and other non-spherical bacteria and Zoogloea with EPS production can play key role in the formation of aerobic granular sludge. The directions of future research as follows should be researched: (1) impacts of various environmental factors on microbial communities components of AGS by using modern molecular ecology such as metagenomics and metatranscriptomics analyses; (2) research on microbial population dynamics of AGS when Accumulibacter , Nitrosomonas and Nitrospira as the main succession of microbial AGS; (3) quorum sensing mechanism of AGS under extreme environment, heavy metal and nano materials; (4) study on the composition and succession of filamentous bacteria in AGS; (5) the relationship among systematic microbial composition, environmental factors and pollutants removal efficiency.